1. Technical Field
This invention relates to a method of adjusting the temperature properties of piezoelectric devices and oscillation circuits, especially a method of adjusting the temperature properties of the SAW device and oscillation circuits where it is mounted.
2. Prior Art
Conventionally, the temperature properties (frequency variation properties against the temperature variation) of piezoelectric resonators are expressed, as shown in FIG. 9, in terms of a second-order function such as in the tuning fork resonator and the SAW resonator in some and a third-order function such as in the AT resonator in others.
In the frequency temperature property of an oscillation circuit using this piezoelectric resonator, the oscillation circuit temperature property and the resonator temperature property are adjusted so that the temperature properties become minimal in the usage temperature range (−40 to +85° C.) centering on the normal usage temperature of 25° C. Normally, in an oscillator which has a piezoelectric resonator with a second-order function temperature property, if it is adjusted so that the peak temperature of the oscillation circuit temperature property becomes located in the center of the usage temperature range, the temperature stability becomes minimum. In a conventional resonator with a second-order function temperature property, the adjustment width of the peak temperature is about 0° C. to 40° C. On the other hand, because the point of inflection temperature cannot be easily adjusted in an AT-cut resonator etc. which have a third-order function temperature property, the first-order coefficient term is generally adjusted so that the oscillation circuit temperature property in the usage temperature range comes to have the minimum width. In actuality, the electrode film thickness and width are adjusted in the former piezoelectric resonator with a second-order function temperature property, and electrode forming direction (X-axis direction) is adjusted so that the rotation is centered on its inflection point in the latter resonator with a third-order function temperature property.
If an oscillation circuit is configured using a resonator with a second-order function temperature property, as shown in FIG. 8, the temperature property of the oscillation circuit is different from temperature property of the resonator in general. The temperature property seems to rotate in the clockwise direction or counter-clockwise direction. This is because the first-order coefficient of the temperature property varies. Although this variation of the temperature property contains not only a variation in the first-order coefficient but also a variation in the higher-order coefficients, it can be represented by a variation in the first-order coefficient in general. If the oscillator frequency and the temperature properties are minimized, as stated above, the peak temperature of the second-order function temperature property is adjusted to the center of the usage temperature range in general. However, the adjustment range of the peak temperature of a resonator is not free, and the adjustable range is limited.
On the other hand, if an oscillation circuit is configured using a resonator with a third-order function temperature property, as shown in FIG. 7, the temperature property of the oscillation circuit is different from the temperature property of the resonator in general. In the same way as in the case of the second-order property, the temperature property seems to rotate in the clockwise direction or counter-clockwise direction. In the case of the third-order function temperature property, moving the point of inflection temperature is difficult. Therefore, if the oscillator frequency and the temperature properties are minimized, it is arranged so that the minimum temperature property in the usage temperature range is obtained by adjusting the first-order coefficient of the resonator in general.
If the usage temperature range of an oscillator is mis-distributed relative to the temperature property adjustment range of the resonator, especially if the peak temperature of the second-order function temperature property cannot be set to the center of the usage temperature range, or if the point of inflection temperature of the third-order function temperature property is off the center of the usage temperature range, it is difficult to adjust so that the temperature property comes to have the minimum temperature width in the usage temperature range. Also, the SAW resonator normally has a second-order function temperature property, and as a method of improving the temperature property of the oscillation circuit, there is only one method where the peak temperature is brought to the center of the usage temperature range.
This invention has an objective of providing a method of adjusting the temperature properties of piezoelectric devices and oscillation circuits which can make the temperature properties of oscillation circuits flatter in the usage temperature range by using the property in the lower temperature range than the point of inflection of resonators with third-order function temperature properties.